The invention relates to dispensing volumes of liquids.
Devices called “fraction collectors” are employed for collecting liquid samples in suitable vessels and/or funnels when eluated from a chromatographical column, where the largely continuous streams of liquid must be distributed over the various collection positions. Hollow needles are usually employed for injecting liquids into the vessels or funnels. A liquid buffer, having a volume ranging from 3 μl to 20 μl, may be formed on the tip of the hollow needle in order to counteract the surface tension of the liquid. If, for the types of applications involved, flow rates are low, liquid droplets adhere to the tips of the needle, instead of dropping off. This may have significant influence on the desired distribution within the addressed vessels or funnels. Consequentially, decisions regarding where these liquid droplets should be deposited must be made and the volume of the droplet needs to be controlled. This is preferably done by generating well defined initial and final conditions for the desired fractions.
The following four droplet-deposition methods that allow generating more or less well-defined starting or initial conditions have become known to date:
Under a first method, the tip of the needle is dipped in a liquid contained in a vessel, where it may, however, happen that the side wall of the needle will be wet by the liquid and become contaminated. This means that some of the liquid involved will be transported to the next fraction, well known as ‘carry over’ which must be avoided, particularly in the case of low flow rates.
A second method that is frequently employed involves depositing droplets on the base of vessels or on the surface of liquids contained in vessels. This method is particularly employed in the case of small sample vessels, such as micro titer plates (well plates) or matrix-assisted laser desorption (MALDI) targets employed in bioanalyses. Of course, depositing droplets on the base of vessels will be possible only if their base has not yet been totally covered with liquid. Other means for depositing droplets on a bounding surface will have to be employed as soon as the latter status has been reached. If the bounding surface involved is the surface of a liquid contained in a vessel, the distance between the tip of the needle and that surface will have to be accurately controlled, and should ideally be adjusted to suit the flow rate and type of liquid involved. Furthermore, the needle will have to be raised in synchronism with the rising levels of liquid in the vessels. This is only possible if all past conditions of the affected liquid/solid surfaces are known. Depositing droplets on the base of sample-collection vessels will, of course, be possible only if the hollow needle employed is sufficiently long. For a given needle length, depositing droplets on the base of sample-collection vessels may thus prove impossible if the vessels involved are too tall.
Under a third method, droplets that form on the tip of the needle are wiped off on a side wall of collection vessels. This, however, requires additional manipulation of the needle and is inapplicable to vessels whose walls taper upward. Another disadvantage of this method is that liquid droplets that have been deposited in this manner remain clinging to the walls of vessels instead of draining down into the bodies of liquid contained therein.
Under a fourth method, droplets that remain clinging to the tip of the needle may be wiped off on prepunctured membranes. Under this method, droplets will remain clinging to the upper surfaces of the membranes and, in addition to the aforementioned disadvantages, there is danger that the side wall of the needle might become contaminated.
In the case of fraction collectors employed for distributing and collecting streams of liquids flowing at relatively high volume flow rates or big sample volume respectively, 3/2-way valves have thus far been preferably employed for diverting liquid to a waste-collection vessel while the needle is in transit between vessels, funnels and/or during waiting periods preceding the start of sample collection. However, employment of such valves is subject to limitations in the case of microfraction collectors, i.e., in cases where streams of liquid flowing at microvolumnar flow rates ranging from, for example, 1 μl/min to 100 μl/min, are to be distributed and collected. The main limitations accure due to the increased external band broadening and the excess volume between valve and needle tip.